I wanted to go, only I couldn't.

I wanted to go, except (that) I couldn't.

According to my dictioanry, except and only used as conjunctions are informal. My dictionary has led me astray before, so I turn to you and hopefully your more authoritive dictionaries for help.

Incidentally, how would you punctuate the above? Period, dash etc?

I wanted to go, but I couldn't.
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Anything to add re the question?
I don't understand - you asked, what is the correct expression, and how is it punctuated, which is what I gave you.
I didn't ask what was the correct expression because I assumed they were both right. Online dictionaries confirm both are used as conjunctions.

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Yes -but the point is, which is most appropriate in the sentence.

I think it best that I don't respond to your posts in future.